1. Field of the Invention
The present invention relates to a thin film transistor and a manufacturing method thereof. Further, the present invention relates to a display device and an electronic device which include the thin film transistor and to which the manufacturing method of the thin film transistor can be applied.
2. Description of the Related Art
In recent years, thin film transistors (TFTs) each including a thin semiconductor film (with a thickness of approximately several nanometers to several hundreds of nanometers) over a substrate having an insulating surface (e.g., a glass substrate) have been attracting attention. The development of TFTs, for example, as switching elements of a display device such as a liquid crystal display device and the like has been accelerated. For such TFTs, an amorphous semiconductor or a polycrystalline semiconductor is mainly used. In addition, TFTs in which a microcrystalline semiconductor is used are also known (e.g., Patent Document 1). In a display device, switching characteristics of the mounted TFTs have an influence on display quality, power consumption, and the like.
One of the parameters that determine switching characteristics of a TFT is an on/off ratio of current. The on/off ratio of current can be increased by increasing on-state current and reducing off-state current.
In this specification, the “on/off ratio” refers to a ratio of on-state current to off-state current in a transistor. Note that the “off-state current” refers to drain current at the time when the TFT is OFF while the “on-state current” refers to current flowing between the source and the drain when the TFT is ON. Further, in this specification, the “drain current” refers to current flowing between the source and the drain.
As a path of the off-state current, a path that is from one of the source electrode and the drain electrode to the other thereof through a semiconductor layer can be given. This off-state current can be reduced by provision of a sidewall insulating layer that is in contact with a sidewall of the semiconductor layer (e.g., Patent Document 2).
However, a favorable TFT cannot be obtained only by realizing a high on/off ratio. For example, it is also important to reduce light-induced leakage current. Here, light-induced leakage current refers to current which flows between the source and the drain due to a photovoltaic effect when light reaches a semiconductor layer of the TFT. Particularly, since a TFT which is used as a pixel transistor of a liquid crystal display device receives light from a backlight on a substrate side, light-induced leakage current should be sufficiently small. For this reason, there have been a lot of developments in a technique for shielding the semiconductor layer of the TFT from light (see Patent Document 3, for example).